Inkjet printers are known that use printheads having a piezoelectric element. With these printers, it is known to print dots of different diameters by applying voltages of different levels to the piezoelectric element. FIG. 35 is a graph showing an example of the relationship between dot diameter and the applied voltage for a conventional approach. For this example, printing was performed for nine levels of dot diameters from small to large. At all levels, a voltage was applied to the piezoelectric element for an interval of 40 .mu.sec. The voltage was increased in conjunction with the increase in gradation level. Accordingly, dots of different diameters can be printed by a single printhead.
To further explain the example shown in FIG. 35, a voltage of 5V was applied at level 1, a voltage of 10 V was applied at level 2, a voltage of 15V was applied at level 3, a voltage of 20V was applied at level 4, a voltage of 25V was applied at level 5, a voltage of 30V was applied at level 6, a voltage of 40V was applied at level 7, a voltage of 50V was applied at level 8, and a voltage of 60V was applied at level 9. At each of the 9 levels the above described voltage was applied for a 40 .mu.sec interval. As can be seen from the graph, the size of the printed dots sequentially increases from level 1 to level 9.
There are three disadvantages described below which are associated with the conventional art.
First, as illustrated by the data in FIG. 35, it is necessary to apply a high voltage to the piezoelectric element to print large diameter dots. It is generally difficult, however, to control the voltage value in the high voltage range. There is concern, therefore, that the waveform will become blunted and result in unstable printing. Furthermore, high-voltage electric circuits and voltage boosters are expensive and increase the cost of the inkjet printhead device.
Second, when printing small diameter dots, a variation may occur in the applied voltage. The diameter of the airborne ink drop is sensitive to and is changed by this voltage variation. Additionally, the variation in ink drop diameter is made more apparent by the spreading of the ink drop after it adheres to the sheet being printed. Accordingly, dot diameter dispersion is a severe disadvantage when printing dots of identical diameter.
Third, in the conventional art, various dot irregularities occur in printing including satellite dots, curves, dot division and the like, due to the difficulty of achieving proper airborne delivery of the ink.